1. Field of the Invention
The present invention relates to an operational amplifier having an offset cancel function, and more particularly to an operational amplifier having an offset cancel function capable of reducing the period until the output terminal changes to a target potential.
2. Description of the Related Art
Operational amplifiers using MOS transistors have a pair of input transistors connected with a common source terminal, a constant current source, which is connected with this source terminal, a current mirror circuit, which is connected respectively with the drain terminals of the input transistors, and, of the pair of input transistors, the drain terminal of one input transistor is connected with the gate of an output transistor, and the drain of the output transistor is connected with an output terminal, such that, when an input voltage is applied to the gate of one input transistor, the output terminal is fed back to the gate of the other input transistor.
In an operational amplifier of this kind that uses MOS transistors, according to an ideal operation of same, when the gate voltages of the pair of input transistors are equal, the drain currents thereof are then equal and stable. Therefore, in a stable state, the input voltage is generated at the output terminal. As a result, the operational amplifier is able to drive a high capacity load connected with the output terminal. Also, it is possible to provide the output terminal with a voltage that is equal to an input voltage. In other words, there is a very great need to equalize the input voltage and the output voltage.
However, when a variation is produced in drain current characteristics with respect to the gate source voltages of the pair of input transistors, even if the drain currents of the two input transistors are in an equal state and are stable, as a result of the variation in the above drain current characteristics, a slight voltage difference occurs in the gate voltages of the two input transistors. In other words, an offset is produced between the input voltage and the output voltage.
The production of an offset voltage of this kind is unfavorable with regard to the operational amplifier characteristics. For example, in a source drive circuit of a liquid crystal display device, in which a plurality of operational amplifiers drive a plurality of source lines, each operational amplifier output voltage (pixel drive voltage) is different with respect to the same input voltage (adjusted level), and causes a color irregularities in the display image. It is therefore necessary to suppress an operational amplifier offset.
As an operational amplifier that suppresses this offset, the present applicant has proposed a novel operational amplifier that is capable of canceling such an offset. For example, such an operational amplifier is disclosed by the Japanese Patent Application 2000-105980 constant on Apr. 7, 2000.
In the proposed operational amplifier, an offset cancel circuit is connected with a pair of input transistors. In concrete terms, a pair of offset cancel transistors and a constant current source are provided in parallel with a pair of input transistors, and an offset cancel capacitor is connected with the gate of one of these offset cancel transistors.
Further, in the offset charge period, the gates of the pair of input transistors are both connected with the input terminal, and the input terminal is connected respectively with the gate of one offset cancel transistor, an output terminal being connected with the gate of the other offset cancel transistor. In this state, drain currents of an equal gate voltage application state are generated in the pair of input transistors, and, the total of the drain currents of the input transistors and the drain currents of the offset cancel transistors connected in parallel therewith is stabilized in a balanced state. Consequently, a voltage value in this stable state is stored in the offset cancel capacitor and an offset drain current is stored in the offset cancel circuit.
Thereafter, the output terminal is isolated from the gates of the offset cancel transistors, and the input terminal and the output terminal are connected respectively with the pair of input transistors, to thereby establish a normal operational amplifier operational state. In this operational amplifier operation stable state, due to the fact that the offset cancel circuit restores a drain current in an offset cancel period, stability is achieved because the drain currents of the input transistors are also restored to values for when the input voltage and the output voltage are equal. As a result, the production of an offset voltage caused by a variation in transistor characteristics may be suppressed.
Further, a variety of other offset cancel circuits have been proposed, an example of which is an offset cancel circuit which stores an offset voltage in an offset cancel period, and, in an operational amplifier operational state, subtracts this offset voltage from the output voltage.
However, the present inventors have discovered a problem with an operational amplifier that cancels an offset requires a long period for the drive of the output terminal. In the case of a constitution in which in order to shorten the period until a stable state is established in the offset cancel period in particular, a switch is provided between a drive output terminal and a drain terminal of an output transistor, and the switch is turned OFF in the offset cancel period to thereby isolate the load capacitance of the output terminal from the drain terminal of the output transistor, drive of the load capacitance of the output terminal begins after the offset charge period has elapsed, thus necessitating a long period until the output terminal ultimately reaches the input voltage.
In other words, in the case of the above LCD source drain circuit, when, while the pixel gray scale level changes, the input voltage changes from an L level to an H level, the response of the output terminal is delayed comparing to the change to the input voltage, meaning that an LCD with a poor response characteristic can be expected.
The problem of the delaying of the output terminal drive time due to this offset cancel period, is not limited to an offset cancel circuit that stores a drain current, but also arises in an offset cancel circuit that stores an offset voltage.
It is therefore an object of the present invention to provide an operational amplifier equipped with an offset cancel function that exhibits a more rapid output terminal response characteristic.
Also, it is another object of the present invention to provide an MOS transistor operational amplifier equipped with an offset cancel function and that exhibits a more rapid output terminal response characteristic with respect to a change in the input voltage.
In order to resolve the above-mentioned objects, according to a first aspect of the present invention, an operational amplifier, which generates an output voltage at an output terminal that is equal to an input voltage, comprises: a differential circuit, which compares the input voltage and the output voltage; first and second output transistors, which are controlled by the output of the differential circuit to drive the output terminal; and an offset cancel circuit, connected with the differential circuit, for storing an offset amount of this differential circuit, wherein, in the offset cancel period in which the offset amount is stored by the offset cancel circuit, the output terminal is driven by the second output transistor, and in the operational amplifier operation period following the offset cancel period, the output terminal is driven by the first output transistor.
In order to resolve the above-mentioned objects, according to a second aspect of the present invention, the operational amplifier comprises: (1) a differential circuit, which has first and second input transistors whose sources are connected, a current source connected with the sources, and a current mirror circuit respectively connected to the drains of the first and second input transistors; (2) a first output transistor, which has a gate controlled by the drain of the second input transistor that is supplied with an input voltage, and a drain that is fed back to the gate of the first input transistor during operational amplifier operation; (3) an offset cancel circuit, which is connected with the differential circuit and which stores the offset state of the first and second input transistors; and (4) a second output transistor, which is provided in parallel with the first output transistor.
In the offset cancel period, the offset cancel circuit stores the offset state, and the drains of the first and second output transistors are isolated such that the output terminal is driven by the second output transistor. In addition, in the operational amplifier operation period following the offset cancel period, the output terminal is driven by the first output transistor. Therefore, while the offset cancel circuit stores an offset value in the offset cancel period, the output terminal is driven by the second output transistor toward a target voltage. Consequently, in the operational amplifier operation period following the offset cancel period, the first output transistor has only to drive the voltage of the output terminal, which has been driven to a value close to the target voltage, to the ultimate input voltage. As a result, the time required for the voltage of the output terminal to ultimately reach the input voltage is shortened.
According to the preferred embodiment of the invention described hereinabove, in the offset cancel period, the drain of the second output transistor and the drain of the first output transistor are isolated such that the output load is driven by the second output transistor, and, in the operational amplifier operation period, the second output transistor assumes a non-active state such that the output load is driven by the first output transistor. Therefore, by making the second output transistor non-active in the operational amplifier operation period, the offset cancel state circuit constitution using the first output transistor of the offset cancel period can be maintained as is even in the operational amplifier operation period.
Further, in a preferable embodiment, a first output current transistor is provided between the drain of the first output transistor and a power source, and a second output current transistor is provided between the drain of the second output transistor and the power source, and wherein the ratio in size between the first output transistor and the first output current transistor, and the ratio in size between the second output transistor and the second output current transistor are made equal. As a result of such a constitution, also after a connection is made between the drains of the first and second output transistors in an operational amplifier operational state, the circuit characteristics, which assumed a stable state in the offset cancel period, can be kept the same, so that it is possible to maintain the offset cancel state.